Urea plant

ABSTRACT

The invention relates to a urea plant with a CO 2  and a NH 3  feed, which comprises a purge line, characterized in that the purge line is connected with a fuel gas input line of a utility plant or an NH 3  plant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 13/805,633,having an international filing date of 24 Jun. 2011, which is thenational phase of PCT application PCT/NL2011/050458 having aninternational filing date of 24 Jun. 2011, which claims benefit ofEuropean patent application Nos. 10167180.8 filed 24 Jun. 2010, and10168065.0 filed 1 Jul. 2010. The contents of the above patentapplications are incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

In commercial urea processes, urea (NH₂CONH₂) is produced by reactingammonia (NH₃) and carbon dioxide (CO₂) at elevated temperature andpressure according to the reactions:

2NH₃+CO₂→NH₂COONH₄

NH₂COONH₄→NH₂CONH₂+H₂O

An overview of commercial processes using this chemistry is given e.g.in Ullmann Encyclopedia, 2005 Wiley-VCH verlag, Weinheim, Germany,chapter Urea. All processes use NH₃ and CO₂ as feedstock. Thesefeedstocks, usually both originating from a ammonia plant containimpurities. The impurities do not react in the abovementioned chemistry,and therefore need to be purged from the urea plant in order to preventtheir accumulation.

The invention relates to a urea plant comprising synthesis equipment,synthesis piping, a CO₂ and a NH₃ feed, and a purge stream. In thispurge stream, part of, or all inert gases that are present in the CO₂and NH₃ feed, as well as part, or all, of any other inert gases fed tothe urea plant are released from the urea plant. Inert gases, in thiscontext are defined as any gaseous components that do not contribute tothe chemical production of urea.

Both feedstocks are usually originating from an ammonia plant. Althoughan ammonia plant is a net heat producer and a urea plant is a net heat(steam) consumer, and it is normal practice to integrate the steamsystems of both plants, net heat is required, which is generallyobtained from burning fuel.

In order to reduce the production of greenhouse gases caused by burningfuel, it is a purpose of the invention to reduce the fuel consumptionrequired for the production of urea.

This purpose is obtained by connecting the purge line with a fuel gasfeed of a utility plant or an ammonia plant, the fuel consumption can bereduced by 2%, which for an average urea/ammonia complex correspondswith a saving of about 7×10⁶ kg natural gas/year.

The NH₃ feed typically comprises also minor amounts of CH₄.

Methane (CH₄) contributes to the growing global background concentrationof tropospheric ozone (O₃), an air pollutant associated with prematuremortality. Methane and ozone are also important greenhouse gases.

A further advantage of the invention is that methane emissions of theurea plant are reduced, which decreases surface ozone and slowing globalclimate warming.

The CO₂ feed is generally provided with an additional oxygen stream,generally originating from air. The oxygen serves as an agent to preventexcessive corrosion of the synthesis equipment and the synthesis piping.As the oxygen does not contribute to the production of urea, it isvented with the purge gas. Excessive corrosion is prevented when theoxygen concentration in the purge gas is in the range of 5-20 mol %

As the NH₃ feed and the CO₂ feed comprise minor amounts of H₂, the purgestream is very likely to be inflammable even before the addition of anoxidizing agent (e.g. air). Because of this inflammable character of thepurge gas stream, it is unsafe to transport this gas (e.g. viapipelines) over some distance. In contrast, it is common practice up tonow to vent this purge gas via shortest possible connections into theatmosphere.

By the use of a duplex ferritic-austenitic steel with a high content ofCr and N and a low content of Ni, as described in WO9500674, as amaterial of construction for the synthesis equipment and synthesispiping, oxygen needs no longer be supplied to the synthesis to preventcorrosion, or only in very low concentrations in the carbon dioxide feede.g. <0.05 vol % of oxygen. Said duplex ferritic-austenitic steel ispreferably a duplex, stainless steel alloy that contains, in % byweight: 0-0.05 C; 0-0.8 Si; 0.3-4 Mn; 28-35 Cr; 3-10 Ni; 1.0-4.0 Mo;0.2-0.6 N; 0-1.0 Cu; 0-2.0 W; 0-0.010 S; 0-0.2 Ce, the remainder beingFe and normally occurring impurities and additives, the ferrite contentbeing 30-70% by volume.

The use of said duplex ferritic-austenitic steel allows a reduction ofthe additional oxygen stream in the CO₂ feed such that the oxygenconcentration in the purge gas can be reduced to between 0-10 mol % or0-1 mol %, without the risk of excessive corrosion taking place.Preferably oxygen is essentially absent in the purge gas. It issurprising that the purge gas stream, essentially without oxygen, nowcan be transported (e.g. by pipelines) without associated safety risks,to be used as a fuel gas.

The purge line of the urea plant of the invention is connected with afuel gas input line of a utility plant or an NH₃ plant. Preferably thepurge stream is directed to the reformer section in an ammonia plant, orto the fuel gas supply of a steam boiler.

The invention will be elucidated hereinafter on the basis of FIG. 1,without being restricted to this embodiment.

FIG. 1 shows a urea plant (1) comprising a CO₂ feed (10) and a NH₃ feed(11). Solid urea leaves the plant via line 12. Water is purged via line13. Purge line (14) is connected with a fuel gas input line of a utilityplant or an NH₃ plant (2).

1. A method to reduce fuel consumption of an ammonia plant or a utilityplant, which method comprises feeding a purge gas stream from a ureaproduction plant, said urea production plant comprising a CO₂ feed andan NH₃ feed, into a fuel gas input line of said ammonia plant or utilityplant; wherein said purge gas stream comprises impurities from said CO₂feed and NH₃ feed.
 2. The method of claim 1, wherein the purge gasstream contains oxygen at a concentration in the range of 0-10 mol %. 3.The method of claim 2, wherein the purge gas contains oxygen at aconcentration in the range of 0-1 mol %.
 4. The method of claim 3wherein oxygen is essentially absent in the purge gas stream.
 5. Themethod of claim 1, wherein the purge stream is fed into a reformersection in said ammonia plant.
 6. The method of claim 1, wherein thepurge stream is fed into a fuel gas supply of a steam boiler in saidutility plant.
 7. The method of claim 1, wherein the CO₂ feed comprisesH₂, wherein the NH₃ feed comprises H₂ and wherein the CO₂ feed comprisesless than 0.05 vol % of oxygen.
 8. The method of claim 1, wherein theurea production plant comprises synthesis equipment and synthesis pipingand the material used for the synthesis equipment and synthesis piping aduplex ferritic-austenitic that contains, in % by weight: 0-0.05 C;0-0.8 Si; 0.3-4 Mn; 28-35 Cr; 3-10 Ni; 1.0-4.0 Mo; 0.2-0.6 N; 0-1.0 Cu;0-2.0 W; 0-0.010 S; 0-0.2 Ce, the remainder being Fe and normallyoccurring impurities and additives, having a ferrite content 30-70% byvolume.
 10. The method of claim 1 wherein said purge gas stream isderived solely from said urea production.
 11. A urea production process,wherein urea is produced by reacting ammonia (NH₃) and carbon dioxide(CO₂), carried out in a urea plant comprising a CO₂ and a NH₃ feed,wherein the plant further comprises a purge line for a purge gas stream,wherein said purge gas stream comprises impurities from said CO₂ feedand said NH₃ feed, wherein the purge line is connected with a fuel gasinput line of a utility plant or an NH₃ plant, and wherein the methodcomprises using the purge gas as a fuel gas in said utility plant or NH₃plant.
 12. A urea production process according to claim 11, wherein theoxygen concentration in the purge gas is in the range of 0-10 mol %. 13.A urea production process according to claim 11, wherein the oxygenconcentration in the purge gas is in the range 0-1 mol %.
 14. A ureaproduction process according to claim 13, wherein the CO₂ feed comprisesH₂, wherein the NH₃ feed comprises H₂, and wherein the CO₂ feedcomprises less than 0.05 vol % of oxygen.
 15. A urea production processaccording to claim 14 wherein the purge stream is directed to thereformer section of said NH₃ plant.
 16. A urea production processaccording to claim 14, wherein said utility plant comprises a steamboiler, and wherein the purge stream is directed to the fuel gas supplyof said steam boiler.
 17. A urea production process according to claim11, wherein oxygen is essentially absent in the purge gas.
 18. A ureaproduction process according to claim 11, wherein the plant comprisessynthesis equipment and synthesis piping wherein as material for thesynthesis equipment and synthesis piping a duplex ferritic-austenitic isused that contains, in % by weight: 0-0.05 C; 0-0.8 Si; 0.3-4 Mn; 28-35Cr; 3-10 Ni; 1.0-4.0 Mo; 0.2-0.6 N; 0-1.0 Cu; 0-2.0 W; 0-0.010 S; 0-0.2Ce, the remainder being Fe and normally occurring impurities andadditives, the ferrite content being 30-70% by volume.
 19. A processaccording to claim 11, wherein said purge line is configured to conductpurge gas derived only from said urea production.